Vibration isolation systems for suspension applications typically incorporate combinations of pneumatic, mechanical, and hydraulic components. These components, in combination, must provide a suspension system function that controls a suspended mass dynamic state with position, natural frequency, and damping characteristics necessary for optimal performance.
Exemplary patents of various suspension systems that incorporate pneumatic components include the following patents and patent applications:
U.S. Pat. No. 3,822,908 issued on Jul. 9, 1974, to Rene Gourand describing a suspension system using single or dual air bags having a tapered cross section. The air bags are pressured by an air pump.
U.S. Pat. No. 4,415,179 issued on Nov. 15, 1983, to Joseph A. Marinelli describes an axle and air bag suspension comprising an air spring in conjunction with a front-to-rear trailing mechanism.
U.S. Pat. No. 4,497,078 issued on Feb. 5, 1985, to Jerald M. Vogel et al. describes a purely pneumatic, three-degree-of-freedom isolation system for a sleeper bunk in a truck. The system incorporates three air springs, each with an accumulator for setting natural frequency, and a directional orifice positioned between the air spring and corresponding accumulator that provides two natural frequency settings for each degree of isolation freedom.
U.S. Pat. No. 4,733,876 issued on Mar. 29, 1988, to Merle J. Heider et al. describes a leaf spring supplemented with a pressure controllable air bag supplying variable spring adjustment, variable ride height, and leveling control of an RV.
U.S. Pat. No. 4,923,210 issued on May 8, 1990, to Merle J. Heider et al. describes a leaf spring in conjunction with an air spring for vehicle leveling function. A pneumatic controller directs the air spring state.
U.S. Pat. No. 5,083,812 issued on Jan. 28, 1992, to Donovan B. Wallace et al. describes an air spring suspension for a vehicle for preventing vehicle roll motions.
U.S. Pat. No. 5,265,907 issued on Nov. 30, 1993, to Ray Tostado describes a bolt on auxiliary air spring suspension that assists a factory suspension.
U.S. Pat. No. 5,346,246 issued on Sep. 13, 1994, to Cecil Lander et al. describes an air spring suspension system controller for setting spring rates in conjunction with a leaf spring system.
U.S. Pat. No. 5,584,497 issued on Dec. 17, 1996, to Cecil Lander et al. describes an air spring controller for automatic adjustment of spring rates on a coupled pneumatic/mechanical leaf spring system.
U.S. Pat. No. 5,765,859 issued on Jun. 16, 1998, to Corbett W. Nowell et al. describes a kneeling wheeled suspension system utilizing air springs for lowering truck trailer decks.
U.S. Pat. No. 5,908,198 issued on Jun. 1, 1999, to Ervin K. VanDenberg describes a center beam and air spring suspension system mounted to a suspension frame and providing varying spring rate capabilities.
U.S. Pat. No. 5,988,672 issued on Nov. 23, 1999, to Ervin K. VanDenberg describes an air spring suspension system for an axle application having horizontal, vertical, and axial spring rates.
U.S. Patent Application Publication No. US 2004/0061293 A1, issued on Apr. 1, 2004, to James M. Barbison describes an air suspension system for an RV that comprises air springs and mechanical dampers that provide vehicle leveling function, as well as vehicle ride suspension on road traversing.
U.S. Pat. No. 6,725,983 B2 issued on Apr. 27, 2004, to Stephen H. Bell describes a shock absorber that provides variable damping based on load conditions of a vehicle. The shock absorber is fluidly coupled with the suspension system air springs. Damping rate is adjusted to levels dictated by air spring pressure.
U.S. Pat. No. 6,733,022 B2 issued on May 11, 2004, to Curtis S. Bradshaw describes a sprint car suspension comprising an air spring in a swing-arm four bar linkage mechanism. The air spring further possesses a non-linear spring rate for keeping the vehicle tires firmly on the ground.
U.S. Patent Application Publication No. US 2004/0178587 A1, issued on Sep. 16, 2004, to Grant W. Hiebert et al. describes an air suspension system for an RV that provides a variable, but discrete, ride quality. Vehicle suspension corner components consist of a pair of air springs connected with an anti-dive valve that allows suspension operation using a single air spring or both, thus yielding two stiffness rates, as needed. A controller and accelerometers are used to trigger the anti-dive valve.
U.S. Patent Application Publication No. US 2005/0098399 A1, issued on May 12, 2005, to Ronald D. Bremner describes an active seat suspension system comprising an air spring with fixed accumulator for providing a natural frequency consistent with good ride quality, variable viscous damper for damping control, and a hydraulic ram system for providing seat height control and canceling base accelerations.
Traditional vibration isolation system used to isolate a suspended mass, m, from potential vibrations consists of a mechanical spring with stiffness k, and a damping mechanism with damping coefficient c (FIG. 1). The two parameters, c and k, are adjusted to provide a “best” isolation environment for the suspended mass, normally based on a transmissibility consideration. Hence, a single operational design point is addressed in this isolation system. Excitation frequencies outside the range covered in the design point definition result in a less than satisfactory vibration isolation environment. This phenomenon is readily understood in the following transmissibility analysis for the system.
The dynamic equation of motion for the suspended mass is given by{umlaut over (x)}+2ζωn({dot over (x)}−ż)+ωn2(x−z)=f(t)  (1)
where the damping ratio, ζ, and natural frequency, ωn, are functions of the system parameters c, k, and m. The transfer function for the system is given by
                                          x            ⁡                          (              s              )                                            z            ⁡                          (              s              )                                      =                              G            ⁡                          (              s              )                                =                                                    2                ⁢                                                                  ⁢                                  ζω                  n                                ⁢                s                            +                              ω                n                2                                                                    s                2                            +                              2                ⁢                                  ζω                  n                                ⁢                s                            +                              ω                n                2                                                                        (        2        )            
The transmissibility is defined to be the ratio of the g-load generated by the base to the g-load experienced by the suspended mass which can be written as follows:
                    Transmissibility        =                                                            s                2                            ⁢                              x                ⁡                                  (                  s                  )                                                                                    s                2                            ⁢                              z                ⁡                                  (                  s                  )                                                              =                                                    2                ⁢                                  ζω                  n                                ⁢                s                            +                              ω                n                2                                                                    s                2                            +                              2                ⁢                                  ζω                  n                                ⁢                s                            +                              ω                n                2                                                                        (        3        )            
FIG. 2 depicts the transmissibility of the system over a range of excitation frequencies, ω. The curves demonstrate the requirement that isolator natural frequency must be tuned to values significantly lower than the major excitation frequency to which the system is subjected. In fact, the natural frequency of the isolator should be set to its smallest possible value for minimum transmissibility. Additionally, the damping of the isolator should be small, if not zero in minimizing transmissibility. The curves also indicate that isolator performance degrades to unsatisfactory levels when excitation frequencies approach isolator resonance values. In fact, amplitude problems near resonance preclude the utilization of small damping rates desired for off-resonance operation.
A complicating factor in the consideration of small natural frequency suspension systems arises from the fact that suspended mass static load deflection is inversely proportional to the spring rate, k. The minimum feasible natural frequency for a suspension isolator is, to a great extent set by suspended mass static deflection demands, thus limiting its transmissibility quality.
In summary, the traditional mechanical spring/damper isolator system is limited to a single design point with performance limitations imposed by other constraints that must be simultaneously satisfied. Isolator performance degrades rapidly as excitation frequencies shift from design point values to near resonance.